As of last Tuesday, 693 cases of West Nile virus have been reported from 43 states to the Centers for Disease Control and Prevention (CDC). The disease is hitting the country early this year, with the CDC reporting the highest number of cases in the second week of August since the virus was first detected in 1999. Experts say a mild winter and rainy spring, followed by prolonged high temperatures and dry weather have contributed to the spike, by giving mosquitoes ample time to breed and ideal conditions to thrive.

Subsequently, the balmy South is seeing the majority of West Nile deaths and cases this year — an epidemic in Texas led Dallas County to declare a state of emergency and authorize the first aerial spraying of insecticide in nearly 50 years — but the disease is a nationwide problem, with the most recent deaths coming from Michigan and Illinois.

Here’s what you should know about the disease and how to stay protected.

1. What is West Nile virus?
West Nile virus is a disease that infects birds and is then spread by mosquitoes to humans. People can also become infected through blood transfusions and organ transplants. Human cases of the disease typically rise at the end of summer and fall. Most people who contract the virus won’t experience any symptoms at all, but others may develop mild symptoms of West Nile fever or, rarely, the more severe symptoms of neuroinvasive West Nile, including inflammation of the brain and surrounding tissues — the most severe form of the disease. Most of the cases reported to the CDC so far have been neuroinvasive disease: 406 cases versus 287 cases of non-neuroinvasive disease.

Other viruses in the West Nile family of viruses are responsible for dengue, yellow fever and Japanese encephalitis.

2. What are the symptoms of West Nile?According to the CDC, about 80% of people infected with West Nile virus will get off scot free, with no symptoms. About 20% of infected people will come down with mild symptoms of West Nile fever, including fever, headache, fatigue, body pain, skin rash and swollen lymph glands. These symptoms can last anywhere from a couple of days to a few weeks.

Only about 1 in 150 people will develop the most severe form of West Nile disease, which can include West Nile encephalitis or meningitis. Severe symptoms can include high fever, headache, stiffness, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These can last several weeks, and neurological effects — while very rare— can cause permanent damage.

People at the highest risk for severe illness are those over 50 or with serious medical conditions like cancer, diabetes, high blood pressure, kidney disease and organ transplants.

3. How can I prevent infection?
Avoiding mosquito bites is the best way to stay disease-free. The CDC recommends taking measures to protect yourself and your family and to secure your environment. Here are some tips:

Apply insect repellent to exposed skin when going outdoors. Make sure repellents will last for the full time you plan to remain outside; products with more active ingredients will last longer. Don’t let kids handle repellent on their own, and don’t let them get it on their hands. Click here for the repellent ingredients that have been reviewed and approved by the Environmental Protection Agency.

Since mosquitoes can bite through thin clothing, it’s a good idea to spray your clothes (and shoes, bed nets and camping gear) with insect repellents that contain permethrin or another EPA-registered repellent. But don’t apply permethrin directly to your skin or under your clothes.

Wear long-sleeved shirts and long pants outdoors.

Put mosquito netting over infant carriers when going outside.

Install or repair window and door screens to keep mosquitoes out.

Try to stay inside at dawn, dusk and early evening, which are peak mosquito times.

Drain any standing pools of water where mosquitoes can lay eggs and breed. Empty stagnant water from flower pots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels, old tires and cans. Check for clogged gutters and any other hidden containers or areas where water might pool around your home.

4. How is West Nile infection treated?
There is currently no treatment or vaccine or prevent West Nile. In most mild cases, symptoms go away on their own. For severe cases, some individuals may need to be hospitalized to treat their symptoms. Anyone who thinks they may be infected with West Nile virus should contact their doctor.

5. Which regions are hardest hit?
The CDC says that more than 80% of West Nile cases are clustered in Texas, Mississippi, Louisiana, Oklahoma, South Dakota and California — and almost half of those cases are in Texas, with 552 cases and 21 deaths reported in the state so far this year. Dallas County has plans to spray insecticide aerially over the most infected regions, but planes have been grounded by rainy weather.